Evolution still takes thousands of years, no matter how urgent the need for evolution, unless of course this species reproduces once every few hours. If this new species cannot feel pain, it will have a tendancy to slowly kill itself without realising. It could injure itself and not realise, it could exhaust it's body of water, vitamins and minerals without having the need to replenish and it would have no fear of dying as fear is painful. It would die off and the only survivors would most likely be ones that feel pain and have the need to avoid danger, beating the point of a hedonistic imperative. Of course you could somehow modify this being to gain more pleasure from avoiding danger, but that still beats the point of a life without pain. As for this being, more pleasure is pleasure and less pleasure is pain.

The imperative would make the new species unreasonable and it would die off.

A better genetic imperative would be for the standard super human. A very intelligent human, with a strong immune system, the emotional capability for compassion and maturity for assertiveness, super efficient and strong muscles, an organ which produces vast amounts of anti-oxidants slowing aging or maybe even a body which consists entirely of fetal cells to prevent aging and mutilation altogether. This super human would feel pain when something is going wrong, but easily have the capability to survive a painful situation.

The imperative should be to grow a superhuman who feels pain when something is wrong and feels pleasure when it is improving, discoverring and surviving.

Let's look at depression as an example. I sometimes get depressed, and therefore function at a lower level. From a genetic standpoint this can be
good because depression can be adaptive or aid survival. This is good for
for our genes, but not for the person experiencing the depression. If we eradicate depression will that have a negative impact on our survival?
Probably not, perhaps a million years ago but not today.

i wonder, if i was in a constant state of happiness, would i notice at all?
isn't happiness when something is better than usual, or than it used to be?
if everyone was happy all the time, there would be no way of appreciating ones happiness as there would be no comparative sadness.
then happiness would be greater happiness and sadness would be less happiness, as it were, and we are back on square one.

i wonder, if i was in a constant state of happiness, would i notice at all?
isn't happiness when something is better than usual, or than it used to be?
if everyone was happy all the time, there would be no way of appreciating ones happiness as there would be no comparative sadness.
then happiness would be greater happiness and sadness would be less happiness, as it were, and we are back on square one.

I have a devotion to my species, and I don't want humans to lose the traits that gave them their name. Taking away stimulating emotions, such as depression or despair, shatters the motive for advanced work. Pure happiness is impossible for our species, as there are uncountable actions that could end our existence. We have to live roughly, unhappily, because that's the way our species is. Humanity can be scared, blindly stubborn, arrogant, and even depressed, because that's the most human characteristic about us. We could enhance our species, through cybernetic implants or genetic modification, but we can't allow ourselves to evolve so quickly. Relativistic flights and cryogenic freezing will keep humanity preserved.

i.e. We could start a facility dedicated to genetic perfection of the human genome. These people would exist outside of the normal world. Perhaps an underground or underwater city.

I think the benefits of this could far outweigh the negatives. And if we did it this way the rest of the world would conitue to evolve normally, so if something went terribly wrong they would be there to stop it, but if something went terribly right they would exists to spread it to the rest of the world, piece by piece.

Just think about it, put it in the back of your mind. Don't rule it out.

Even if we took away all the nagative emotions, I feel the human spirit would still thrive. We would change the face of humanity. Eliminate prejiduce. Eliminate genetic disease. Enhance our immune systems. Etc...

I can see why some people think this is a bad idea, but I can also see how those people will become less and less with time. We shall see, time will tell...